FIGS. 1A and 1B are diagrams showing a known arrangement for optical biometric sensing, typically for taking an image of the fingerprint of a digit of one hand. These figures show a known example of an optical device for biometric sensing comprising a prismatic optical element 1 (in this case an isosceles trihedral prism) having at least one main face 2 and first and second side faces, 3 and 4 respectively, that are inclined on either side of the main face, at least one portion 5 of said main face 2 constituting an application surface for pressing against a body organ (in particular a finger 6 in FIG. 1B) presenting a biometric characteristic that is to be detected (here the fingerprint of the finger 6). The device also includes main illumination means 7 disposed facing the prismatic optical element 1 remotely from said main face 2 (specifically facing the dihedral ridge formed by the side faces 3, 4) so that the main radiation 8 emitted by said main illumination means 7 penetrates into the prismatic optical element 1 substantially perpendicularly to the main face 2. Furthermore, the device includes sensor means 9 situated facing the first side face 3, with a focusing element (e.g. a converging lens 10) being interposed therebetween. Finally, it should be observed that the side face 4 opposite from the side face 3 facing the sensor means is black (the face 4 is coated in black varnish), as represented by a thick black line.
In the example shown in FIGS. 1A and 1B, said portion 5 constituting an application surface for a body organ is in the form of a window 11 opened in a housing 12 containing the device.
As shown in FIG. 1A, when no body organ is applied to the portion 5, the radiation 8 is reflected in the prismatic optical element 1 towards the black face 4, and by total internal reflection the sensor means 9 receives the image of the black face reflected on the main face 2.
As shown in FIG. 1B, when a body organ 6 is placed on the portion 5, some of the radiation is refracted by the so-called “moist” contact of portions in relief on the body organ 6 (the ridges of the fingerprint), which are the only portions genuinely pressed against the face 2: this makes it possible to perceive in the reflected image the image of the fingerprint (the ridges in contact with the surface of the portion 5 appear in white and the furrows that are not in contact with the surface of the portion 5 appear in black).
Such an optical biometric sensor device can give rise to various frauds, such as in particular injecting a fraudulent biometric image through the window 11, playing back a previously sensed image, physically removing the prismatic optical element 1 and replacing it with a fraudulent element, e.g. having a stolen fingerprint image placed thereon (e.g. drawn or in the form of traces), etc.